In general, professional golfers or advanced golfers like a thread wound golf ball because of its high impact resilience, good hit feeling and excellent spin performances. However, it has hitherto been considered that the thread wound golf ball is not suitable for amateur golfers who require a long flying distance because the flying distance is inferior in comparison with a two-piece golf ball.
In the thread wound golf ball, since the amount of back spin put on the golf ball upon hitting is large, the lifting power is liable to act on the golf ball. In the process immediately after launching at which the velocity of the golf ball is high, the flying power and lifting power of the golf ball is strong and the golf ball launched at a low launching angle is blown up. The velocity of this golf ball is gradually reduced due to air pressure or resistance during flight. At this stage 2, the spinning ratio is reduced and the lifting force of the golf ball is reduced. Accordingly, the trajectory of the golf ball begins to go down rapidly as the lifting force and projectile force decrease. That is, the landing angle is large the rolling distance is small. This was the cause of its low flying distance in comparison with the two-piece golf ball.
In the ideal golf ball, the launch angle is high, the initial velocity is large, the amount of spin is small and a suitable lifting force acts on the golf ball. Therefore, the golf ball reaches the peak point gently without blowing up, and then begins to drop. It is preferred that the amount of spin is large (i.e. the lifting force is large) and the golf ball is not easily dropped at this time.
Heretofore, the weight has been concentrated in the center part to reduce the moment of inertia such that spin is easily put on the golf ball. The diameter of center of the golf ball is merely defined as 25 to 29 mm empirically, as described in "Rubber Industry Handbook". The moment of inertia is small and spin is easily put on the golf ball, and there is a limit in flying distance.
Therefore, a thread wound golf ball which can accomplish a long flying distance has hitherto been required. In this case, it was considered that the initial velocity of the golf ball can be increased by employing a rubber thread having high impact resilience. However, the initial velocity of the golf ball is defined by rules of U.S. Golf Association, and there is a limit in initial velocity.
Further, as described in Japanese Laid-Open Patent Publication 59-129072, there was suggested a trial of increasing the flying distance by decreasing the specific gravity of the center and, on the contrary, increasing the specific gravity of the cover to increase the moment of inertia. However, since an excessive amount of fillers are formulated in the cover material, stiffness of the cover itself becomes high, which results in a hard hit feeling, and it is not preferred. Although the moment of inertia is increased, there is a limit. Further, this technique is for the solid center. In order to reduce the amount of spin appropriately, a liquid center is employed. The liquid center has hitherto been employed for the balata cover golf ball on which spin is easily put. The liquid center itself exhibits no impact resilience but it follows deformation of the golf ball upon hitting. The deformation force applied to the rubber thread around the center thereby affords a restoring force to a rubber thread having high impact resilience.
In Japanese Laid-Open Patent Publication No. 60-92782, there is described that it is effective for decreasing the spin ratio of the golf ball to increase the diameter from the conventional diameter (25.4 to 26.99 mm) to 28.58 mm. Since the amount of spin of the golf ball with the balata cover is large, a trail has been done with respect to increasing the flying distance by controlling the amount of spin.
However, this technique merely discloses a change of the diameter of the center within a narrow range such as 25 to 29 mm. As to the diameter of the center, in Japanese Laid-Open Patent Publication No. 54-4626, there is described that the desirable diameter of the center is 25 to 29 mm. In Japanese Laid-Open Patent Publication Nos. 59-129072 and 48-4025, there is described that the desirable diameter of the center is 30 to 38 mm and 23 to 39 mm, respectively. In the "Rubber Industry Handbook" issued on 1973, page 864, column of golf ball, there is described that the outer diameter of the liquid center is 25 to 29 mm.
In Japanese Laid-Open Patent Publication No. 61-187875, there is suggested a golf ball comprising a liquid center having a specific gravity of 0.7 to 1.3 and a center cover having a specific gravity larger than that of the liquid center, the diameter of said liquid center being 25 to 34 mm. In this golf ball, a center cover has a large specific gravity in order to increase the moment of inertia. On the other hand, in Japanese Laid-Open Patent Publication No. 2-255162, there is suggested that the specific gravity of the center cover is decreased and, as a result, the golf ball becomes soft because the amount of the filler of the center cover is decreased, thereby affording large deformation of the center and a reduction of the amount of back spin on hitting to increase the flying distance.
However, the flying distance was not greatly increased in any golf ball.